Myelofibrosis with myeloid metaplasia is a clonal hematopoietic stem cell disorder that results in progressive cytopenias, splenomegaly, blastic transformation, and death. No broadly applicable therapy is available. The pathogenetic mechanism of MMM is currently unknown. A defect in the normal process of apoptosis has been demonstrated in the related myeloproliferative disorders of chronic myeloid leukemia and polycythemia vera. We have shown that apoptosis (spontaneous, serum deprivation, and TNF-alpha induced) is quantitively diminished in the granulocytes of patients with MMM. We have also observed that erythroid precursors from MMM patients can be grown in vitro in the absence of the prerequisite cytokine erythropoietin. Cytokine independent growth has been characterized in polycythemia Vera to arise from over-expression of Bcl-XL (an anti-apoptotic member of the Bcl-2 family). We believe the diminished apoptosis we have observed in MMM may be linked to cytokine hypersensitivity and, potentially, to the anti-apoptotic pathways of Bcl-2 or the Akt pathway. We hypothesize that apoptosis is dysregulated in granulocytes in MMM, and this is a reflection of the corresponding defect in the aberrant clone. In this grant application we propose to: 1.Compare baseline levels of apoptotic proteins and regulators across the spectrum of MMM patients and controls. Baseline levels of apoptotic proteins (caspases), and regulators (lAP's, Bcl-2 family members) will be assessed across a spectrum of MMM patients and normal controls. 2. Evaluate the regulation of caspase activation in MMM neutrophils subjected to apoptotic stimuli through both cellular and cell free systems. Isolated neutrophils from MMM patients and controls will be subjected to various apoptotic stimuli to delineate which pathway of apoptosis is aberrantly regulated. Subsequent experiments will use both immunoblotting and a cytosol caspase activation assay to determine which caspases and regulators are responsible for the apoptotic defect seen in MMM neutrophils. 3. Evaluate the role of the phosphatidylinositol 3- kinase pathway on cytokine independent growth in myeloid progenitors in MMM. Cytokine independent growth of myeloid colonies will be confirmed across a spectrum of MMM patients. Subsequent experiments will delineate the role of the phosphatidylinositol-3 kinase pathway in both apoptosis resistance and cytokine independent colony growth. Successful accomplishments of these goals will provide the scientific basis for targeted anti-myeloproliferative therapy for the treatment of patients suffering from MMM and potentially related chronic myeloid disorders.